The invention relates to an infared radiation detector. The detector comprises a first pyroelectric detector element having first and second electrodes arranged opposite each other on first and second opposite major surfaces of a pyroelectric body. A second pyroelectric detector element has third and fourth electrodes arranged opposite each other on the first and second major surfaces.
Infrared radiation detectors comprising pyroelectric detector elements may be used for a variety of purposes. For example, they may be used in remote switching systems, in intruder alarms, and in movement sensors generally. Such sensors rely on the fact that a human being naturally provides a moving source of infrared radiation as he walks about or even as he moves only part of his body, for example by waving his hand. The radiation which he emits is converted by the pyroelectric detector into an electric signal which can be used, for example, to actuate an alarm or to switch lights on or off.
In order to provide immunity from undesired signals generated by variations in ambient temperature and background radiation, known detectors comprise two pyroelectric detector elements connected differentially. Ambient temperature changes and background radiation changes affect both elements equally, and so generate equal but opposite signals which cancel each other to give no net output signal from the detector. On the other hand, a moving source of infrared radiation provides an output signal when the two detector elements receive the radiation unequally, for example by using a focussed optical system.
It is common for the two pyroelectric detector elements to be connected electrically in series, but with opposite polarity. For example, in U.S. Pat. No. 3,839,640 two separate electrodes are provided on one side of a uniformly poled pyroelectric plastic film, and a single common electrode is present on the other side of the film opposite the first two electrodes. The common electrode floats electrically and provides the series connection between the two detector elements.
In operation, the electric potential on the floating electrode may reach very high values, especially if the detector is subject to wide temperature variations. Unfortunately, this can be a source of noise because such high potentials tend to drive current through the capacitive pyroelectric element. Also, these high potentials can cause undesired depoling of the pyroelectric material. In the case of very thin elements subjected to rapid temperature changes, there may even be an electrical discharge over the edges of the elements.
The problem of noise caused by the floating electrode can be overcome by connecting the two elements electrically in parallel, but still with opposite polarities. This arrangement generally has a better signal-to-noise ratio than an equivalent series-opposed arrangement.
A parallel-opposed arrangement may comprise two physically separate detector elements with opposite polarity. Each element comprises a piece of pyroelectric material sandwiched between a front and a rear electrode. The two front electrodes are electrically connected together, and the two rear electrodes are electrically connected together, typically with wire leads.
Compared with the series-opposed arrangement described above, the parallel-opposed arrangement has the disadvantage that the two detector elements are not formed of a single piece of pyroelectric material. Hence, it is more difficult to manufacture and assemble. In some, but not all, pyroelectric materials it is possible to make a parallel-opposed arrangement in a single piece of pyroelectric material by poling two different parts of the material in opposite directions. However, this is inconvenient and can induce stresses in the material possibly leading to device breakage.
U.S. Pat. No. 3,877,308 discloses a parallel-opposed detector arrangement. The detector has a single body of uniformly poled pyroelectric material comprising two pyroelectric detector elements. One of the detector elements has first and second opposite electrodes on opposite major surfaces of the body and the other detector element has third and fourth opposite electrodes on the same surfaces. The first electrode is connected to the fourth electrode by a conductive lead and the second electrode is connected to the third electrode by a conductive lead. Unfortunately, this structure has the disadvantage that it is difficult to connect a conductive lead such as a gold wire from an electrode on one surface of the body to an electrode on the opposite surface and, these connections cannot easily be made using conventional wire bonding techniques and apparatus. Moreover, for thin pyroelectric detector elements (that is to say elements whose thicknesses are less than 100 microns), it is even more difficult to make wire bonds because the elements are so fragile that they are prone to break under the pressure of the wire bonding operation.